Stabilization of the spine for various conditions, including degenerative disc disease, scoliosis, spondylolithises and spinal stenosis often require attaching implants to the spine and then securing the implants to spinal rods. Such spinal fixation devices can immobilize the vertebrae and can alter the alignment of the spine over a large number of vertebrae by means of connecting at least one elongate rod to the sequence of selected vertebrae. Such rods can span a large number of vertebrae, such as three or four. However, the spine anatomy rarely allows for three or more implants to be directly in line. In order to allow for this irregularity, the rod must be contoured to the coronal plane. With anatomical curvature in the saggital plane found in the lumbar spine, the rod has to be contoured in both planes, requiring considerable effort and surgical time.
For example, the U.S. Pat. No. 5,554,157, issued Sep. 10, 1996, U.S. Pat. No. 5,549,608 issued Aug. 27, 1996, and U.S. Pat. No. 5,586,984 issued Dec. 24, 1996, all to Errico et al. disclose polyaxial locking screws and coupling element devices for use with rod fixation apparatus. The '157 patent discloses a coupling element including an interior axial passage having an interior surface which is inwardly curvate at the lower portion thereof such that it comprises a socket for polyaxially retaining a spherical head of a screw. The coupling element further includes a pair of vertically oriented opposing channels extending down from the top of the coupling element which define therebetween a rod receiving seat. The channel further provides the walls of the upper portion to a pair of upwardly extending members, each including an exterior threading disposed on the upper most portion thereof for receiving a locking nut. During the implantation of the assembly, the locking nut seats against the top of the rod which in turn seats on top of the screw head. The nut causes the rod to be locked between the nut and screw and the screw to be locked in the socket.
The '608 patent discloses a modification wherein a locking ring is disposed about the exterior of the lower portion of the coupling element and provides an inward force on an outwardly tapered portion upon downward translation thereof, causing the interior chamber to crush lock a screw head therein to eliminate the polyaxial nature of the screw element coupling.
The '984 patent discloses a polyaxial orthopedic device including a cutter element having a tapered lower portion including a slotted interior chamber in which a curvate head of a screw is initially polyaxially disposed. The coupling element includes a recess for receiving a rod of the implant apparatus. A locking ring is disposed about the lower portion of the coupling element and provides an inward force on the outwardly tapered portion upon downward translation thereof. The vertical slots are caused to close and crush, thereby locking the screw head within the inter chamber thereof.
With regard to a posterial spinal fixation system, the size of the implant is an important issue. Bulky components can cause soft tissue irritation, as well as comprise the facet joints at the end of a fusion. Minimizing the size of the implants used is critical in spinal surgery with regard to this issue of bulky components. Soft tissue irritation resulting from extensions of implants is a common occurrence. Many times it is caused by the implant being too high relative to its environment. For example, implants can be too high to sufficiently be covered with muscle tissue. Hence, a reduction in the overall height of an implant is a critical advantage for the patient.
Most spine implants utilize a rod connection mechanism that requires a nut, connector, or other component to secure the implant to the rod, such as in those prior art patents discussed above. This connection means has at least some portion that sits above the rod to which the implants are connected. This increases the overall height of the implant assembly in the body.
Several attempts have been made in the prior art to address this concern of the overall height of the implant assembly. Modulok.RTM. (manufactured by Zimmer and Wright Medical, Arlington, Tenn.), Versalok.RTM. (manufactured by Wright Medical, Arlington, Tenn.), and Paragon.RTM. (manufactured by DANEK, Memphis, Tenn.) are all products in the marketplace utilizing a taper connection mechanism to minimize the amount of material situated above the rod in the spinal assembly. However, these types of designs have various inherent difficulties. The Modulok system requires a tapered sleeve to be snapped in place over the rod prior to locking the rod to the implant. The Paragon system requires two tapered sleeves to be used. The Versalok system, specifically designed for lower back fixation, requires that the rod be snapped into a channel and an outer sleeve be brought up to compress the rod interface.
All of the above-mentioned systems have drawbacks, including the bulky instrumentation needed to engage the tapers which somehow must be brought into the surgical environment, thereby causing various difficulties in surgical use. For example, a surgeon usually determines the contour of a rod by contouring the rod and placing it into the saddle or seat of the implants. Then the surgeon examines the contour, removes the rod, and re-contours the rod as required. The Versalok system, due to the design thereof, requires a trialing system for such a procedure. This adds still another step for the surgeon.
In view of the above, it is desirable to have a rod locking mechanism that allows the rod to be readily inserted and removed from the implant, and critically, the mechanism minimizing the height of the implant. This combination of features alleviates the height problem of the prior art while also resolving the issue of multiple contouring steps that are required during the surgical process.